Liquid crystals currently are used in a wide variety of devices, including optical devices such as visual displays. Such devices usually require relatively low power and have a satisfactory response time, provide reasonable contrast, and are relatively economical. The property of liquid crystals enabling use, for example, in visual displays, is the ability of liquid crystals to transmit light on one hand, and to scatter and/or absorb light, on the other, depending on the alignment (or lack of alignment) of the liquid crystal structure with respect to a prescribed input, e.g. an electric field applied across the liquid crystal material. An example of electrically responsive liquid crystal material and use thereof is provided in U.S. Pat. No. 3,322,485.
The invention of the present application is disclosed hereinafter particularly with reference to the use of liquid crystal material that is particularly responsive to a prescribed input preferably of the electromagnetic type, and, more particularly, to an electric field.
Various principles of the invention may be employed with various one or ones of the other known types of liquid crystal material or combinations thereof. The various characteristics of the cholesteric, nematic and smectic types of liquid crystal material are described in the prior art.
To enhance contrast and possibly other properties of liquid crystal material, pleochroic dyes have been mixed with the liquid crystal material to form a solution therewith. The molecules or structure of the pleochroic dye generally aligns with the molecules or structure of the liquid crystal material. Examples of the use of pleochroic dyes with liquid crystal material are described in U.S. Pat. Nos. 3,499,702 and 3,551,026.
An important characteristic of liquid crystal material is anisotropy. An anisotropic material has different physical properties in different directions. For example, liquid crystals are optically anisotropic i.e. they have indices of refraction which vary with the direction of propagation and polarization of the incident light.
Liquid crystal material also has electrical anisotropy. For example, the dielectric constant for nematic liquid crystal material may be one value when the liquid crystal structure is parallel to the electric field and may have a different value when the liquid crystal structure is aligned perpendicular to an electric field. Since such dielectric value is a function of alignment, for example, reference to the same as a "dielectric coefficient" may be more apt than the usual "dielectric constant" label. Similar properties are true for other types of liquid crystals.
A pleochroic display, i.e. one in which pleochroic dye and liquid crystal material are in solution together, has the advantage of not requiring the use of a polarizer. However, such a pleochroic device has a disadvantage of relatively low contrast when only nematic liquid crystal material is used. It was discovered in the past, though, that a cholesteric liquid crystal could be added to the nematic one together with the dye to improve the contrast ratio. See White et al article, "Journal of Applied Physics", Volume 45, No. 11, November 1974, at pages 4718-4723, for example. The cholesteric material would tend not to return to its original zero field form when the electric field is removed.
As used herein the term "liquid crystal" material may mean[s] the liquid crystals themselves and, depending on context, the pleochroic dye and/or other additives in solution or otherwise included therewith.